Re: [PATCH] fs: btrfs: prevent unintentional int overflow

[David Sterba <dsterba@suse.cz>]

On Fri, Jan 03, 2020 at 01:47:40PM -0500, Cengiz Can wrote:
> Coverity scan for 5.5.0-rc4 found a possible integer overflow in
> tree-checker.c line 364.
> 
> `prev_csum_end = (prev_item_size / csumsize) * sectorsize;`
> 
> Quoting from scan results:
> 
> ```
> CID 1456959 Unintentional integer overflow
> 
> Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN) overflow_before_widen:
> Potentially overflowing expression `prev_item_size / csumsize * sectorsize`
> with type unsigned int (32 bits, unsigned) is evaluated using 32-bit
> arithmetic, and then used in a context that expects an expression of type u64.
> (64 bits, unsigned).
> ```
> 
> Added a cast to `u64` on the left hand side of the expression.
> 
> Compiles fine on x86_64_defconfig with all btrfs config flags enabled.
> 
> Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
> ---
>  fs/btrfs/tree-checker.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
> index 97f3520b8d98..9f58f07be779 100644
> --- a/fs/btrfs/tree-checker.c
> +++ b/fs/btrfs/tree-checker.c
> @@ -361,7 +361,7 @@ static int check_csum_item(struct extent_buffer *leaf, struct btrfs_key *key,
>  		u32 prev_item_size;
>  
>  		prev_item_size = btrfs_item_size_nr(leaf, slot - 1);
> -		prev_csum_end = (prev_item_size / csumsize) * sectorsize;
> +		prev_csum_end = (u64) (prev_item_size / csumsize) * sectorsize;

The overflow can't happen in practice. Taking generous maximum value for
an item and sectorsize (64K) and doing the math will reach nowhere the
overflow limit for 32bit type:

64K / 4 * 64K = 1G

I'm not sure if this is worth adding the cast, or mark the coverity
issue as not important.